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Select and Cases Give Different Answers

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99125] Select and Cases Give Different Answers
  • From: Gregory Lypny <gregory.lypny at videotron.ca>
  • Date: Tue, 28 Apr 2009 04:42:49 -0400 (EDT)

Hello everyone,

Suppose I have the following list that is a mixture of the string "NA" 
and five numeric quantities.

	theList = {"NA", "NA", -0.01315, 0.0120957, -1/41, 0.00625, "NA", 
"NA", 5/8,  "NA"}

I want to weed out the NAs, and all of the following

	Cases[theList, _?NumericQ]

	DeleteCases[theList, _?StringQ]

	Select[theList, # =E2=88=88 Reals &]

	Select[theList, # =!= "NA" &]

work fine because each returns {-0.01315, 0.0120957, -(1/41), 0.00625, 
5/8}.

But I don't understand why Unequal, when used as the criterion for 
Select as in

	Select[theList, # != "NA" &],

fails to return the rationals, and I get only three elements 
{-0.01315, 0.0120957, 0.00625}.  I'd appreciate any clarification on 
the difference between UnsameQ and Unequal because my inclination in 
most circumstances is to use an equal sign or an unequal sign to make 
simple comparisons yet it doesn't give me the answer I expect, and I'd 
hate to make a costly mistake somewhere down the road!

Regards,

	Gregory


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